1. Field of the Invention
The present invention relates to a method of processing seismic data and, in particular, to a method of producing a semblance panel. It also relates to an apparatus for processing seismic data.
2. Description of the Related Art
Once seismic data has been acquired in a seismic survey, it is processed to obtain information about the geological structure of the earth' interior. In order to process seismic data it is necessary to know, or to estimate, the way in which the velocity of seismic energy varies within the earth (this step is otherwise known as determining the “velocity field”). The accuracy of the information obtained about the earth's interior will depend on the accuracy of the velocity field used in the processing of the seismic data.
The step of determining or estimating the velocity of seismic energy from a seismic trace is generally known as “velocity-picking”. In essence this consists of identifying a particular feature in a seismic trace, and assigning a particular path of seismic energy to that feature. The velocity of seismic energy can then be determined from the length of the seismic energy path and the travel time of seismic energy producing that feature.
Velocity-picking is currently carried out manually. It is highly labour intensive and consumes a very significant proportion of the human resources required to process seismic data
Many techniques for automatic velocity-picking using a computer have been proposed. The reliability of such automated velocity-picking methods is poor, however, and manual quality checking of the results of automatic velocity-picking is required. This quality checking may well take nearly as long as doing the velocity-picking manually.
Thus, there is a need for a method of velocity picking that is less labour intensive than current methods, but that is more reliable than current automated velocity-picking methods.
Many current methods of velocity-picking use “semblance panels”. Semblance panels are in general use in the seismic industry as a means of determining velocity fields for further processing of seismic data.
A semblance panel consists of a contour map that has axes of time and velocity, and the vertical axis (that is, the axis perpendicular to the time-velocity plane) represents the semblance. The objective of velocity-picking is to define a function within the time-velocity space that passes through the highest points of the contour map (whilst satisfying constraints such as continuously increasing time and restrictions on velocity inversions). One current method of automatic velocity-picking relies on an iterative technique that attempts to locate the highest points of the contour map by perturbing a ‘seed’ function in such a way as to move it up the flanks of the peaks on the map and, in doing so, maximise the integral under the function.
One of the problems with this current approach is that the method can locate a local maximum, effectively finding a foothill of a peak rather than the peak itself. Such “foothills” are often introduced into the contour map by noise in the seismic data. Once an automated velocity-picking method has mis-directed itself towards a foothill, rather than the principal peak, it will remain stuck on the foothill and will not locate the principal peak. The automated velocity-picking method will thus not pick the correct velocity, and hence manual quality checking of the results is required.
One prior art attempt at improving the accuracy of conventional automated time-picking methods is to reduce the noise in the seismic data This reduces the height of the foothills, and so reduces the chance that the automated velocity-picking method will locate a foothill rather than a principal peak. Using data from other points in the immediate vicinity of the analysis point is one way to increase the volume of data and so improve the signal-to-noise ratio. The problem with this approach is that, if the seismic data being processed was obtained from a dipping reflector (that is, from a reflector that is not horizontal), the semblance maxima from adjacent data points do not occur at the same time. This means that smearing of the semblance peaks will occur when data from more than one analysis point is used, and this smearing will offset any increase in accuracy caused by the increased signal-to-noise ratio. As a result this approach has been used infrequently. Moreover, if this method is used it is used to only a very limited extent, in that data from very few adjacent points can be added together before significant smearing of the semblance maxima occurs.